1. Field of the Invention
The invention pertains to device fabrication methods in which precursor polymeric or oligomeric materials undergo curing reactions involving crosslinking and/or imidization, to form solid, amorphous polymeric materials, as well as the resulting devices.
2. Art Background
Polymeric or oligomeric materials capable of undergoing crosslinking and/or imidization have many commercial applications. For example, a variety of thermosetting epoxies are used as adhesives because they, as well as the catalysts and/or curing agents used in conjunction with the epoxies, are commercially available in liquid form and are thus readily applied to device components which are to be adhered to one another. Upon being heated, a curing reaction is initiated in which epoxy molecules react with, i.e., undergo crosslinking with, the curing agent and/or other epoxy molecules, resulting in the formation of a solid, relatively rigid, amorphous polymeric material, which serves to bond the device components to one another. For example, among the most common of the thermosetting epoxies are the bisphenol A epoxies. When heated in the presence of, for example, a primary amine (a curing agent), a bisphenol A epoxy undergoes crosslinking with the amine, resulting in the formation of a more densely crosslinked polymeric material, i.e., a material containing aminoalcohols (the product of the crosslinking reaction), which material has a solid, relatively rigid, amorphous structure. Moreover, the continued addition of heat leads to crosslinking between aminoalcohol molecules, resulting in a new, even more rigid polymeric material.
Thermosetting epoxy novolac molding compounds are currently being used to form encapsulants for integrated circuits (ICs). In this regard, when encapsulating ICs, a solid pellet, containing a mixture of an epoxy novolac and a curing agent, such as a phenolic novolac, is typically initially heated in a dielectric heater to soften the pellet. Then, while soft, the pellet is transferred to a mold maintained at a relatively high temperature, from which the pellet material is extruded into cavities containing ICs, to encapsulate the ICs. Significantly, some curing of the pellet material, i.e., some crosslinking between the epoxy novolac and the curing agent, takes place within the dielectric heater and within the heated mold. However, the pellet generally spends too little time within the dielectric heater and heated mold to achieve an acceptable degree of cure. Consequently, further curing is achieved by heating the encapsulated ICs, for an additional period of time, within an oven maintained at the same temperature as the heated mold, resulting in the formation of more densely crosslinked, amorphous, polymeric encapsulant.
Polyimides, in the form of thin films, are now in use as insulators in semiconductor devices. Such a polyimide film is typically formed by dissolving a precursor polyamic acid in an organic solvent, which is deposited as a thin film onto a semiconductor device substrate using conventional spin deposition techniques. This film of polyamic acid-solvent solution is then heated to evaporate the solvent and to achieve imidization, resulting in a solid, amorphous, polyimide film.
As noted, the above curing reactions lead to the formation of solid, amorphous, polymeric materials. Because they have amorphous structures, these materials are, in fact, glasses, which are characterized by, among other things, glass transition temperatures, T.sub.g. (In the present context, the glass transition temperature of a polymeric material is that temperature below which the material has a solid, amorphous structure, and above which the material has a soft, liquid-like or rubbery structure.)
A polymeric or oligomeric material capable of undergoing crosslinking and/or imidization is conventionally cured by being heated at a relatively high, constant temperature in order to achieve curing in a relatively short, commercially reasonable time period, e.g., several minutes to several hours. During this curing process, the T.sub.g of the material (which reflects the state of cure of the material) generally increases (as crosslinking and/or imidization proceeds) until it is equal, or almost equal, to the curing temperature. Significantly, because the curing temperature exceeds the T.sub.g of the material throughout most of the curing process, the material is necessarily in a soft, liquid-like or rubbery state during curing.
Until recently, it was believed that the T.sub.g of a conventionally cured, crosslinkable polymeric material could never exceed the cure temperature. (See, e.g., John K. Gillham, "Cure and Properties Of Thermosetting Systems," Proc. 13th North American Thermal Analysis Soc. Conf., Sept. 23-26, 1984, Philadelphia, Pa., pp. 344-347.) This belief has now been disproved. That is, it has been determined that the T.sub.g 's of conventionally cured, crosslinkable polymeric materials, having amorphous, glassy structures, continue to increase well beyond cure temperature when these glassy materials are isothermally aged, i.e., exposed to a constant temperature ambient, where the constant temperature is below the T.sub.g of the material. Such an ambient is, for example, a room temperature, or higher temperature, environment. (See H. E. Bair, "Curing Behavior Of An Epoxy Resin Above And Below T.sub.g ", edited by B. M. Culbertson, ACS Polymer Preprints, Vol. 26, No. 1, Apr. 1985, pp. 10-11.) In this regard, it is now believed that the mechanism, i.e., the rate of crosslinking, associated with conventional curing is primarily related to the concentrations of the reactants and the curing temperature. By contrast, it is now also believed that the mechanism associated with the additional curing achieved via isothermal aging is primarily related to the molecular configuration of the (partially) crosslinked polymeric material, i.e., the rate of additional crosslinking is related to the time scale over which molecular configurational rearrangements occur to allow reaction at crosslinkable sites. Significantly, as evidenced by the rate of increase of T.sub.g, the rate of crosslinking associated with the additional curing of glassy, polymeric materials achieved via isothermal aging, hereafter termed configurational curing, is much lower than the rate of crosslinking achieved during conventional curing. In fact, the configurational curing rate at, for example, room temperature is so low that many thousands of hours are needed to achieve significant additional curing, which suggests that configurational curing is commercially impractical.
Although the curing rate achieved to date with configurational curing is impractically low, configurational curing has one advantage which conventional isothermal curing does not--the material undergoing curing remains relatively rigid. This is particularly advantageous in those instances where, for example, a first device component is to be adhered to a second device component with, for example, a thermosetting epoxy, after being aligned with a third device component. If the alignment is to be maintained, then the epoxy must necessarily remain rigid during curing.
As a consequence, those engaged in developing applications for polymeric or oligomeric materials capable of undergoing crosslinking and/or imidization have sought techniques for curing or further curing these materials, via configurational curing, in commercially reasonable periods of time.